The original statement of the law comes from section 2.10 of RFC 793
which can be found here:

http://www.ibiblio.org/pub/docs/rfc/rfc793.txt

The actual quote is:

  TCP implementations will follow a general principle of robustness:  be
  conservative in what you do, be liberal in what you accept from
  others.

Clearly at the the time Postel meant it to apply only to TCP.

not obvious to me that he was referring only to spec interpretation
here. Perhaps that becomes clear when this is read in the context of
the whole TCP spec. But whether he meant it or not, the weak form of
Postels' law is a lot more palatable to me than the strong form.